9 Fundamentals of the Application of Simulation in the Forging Industry All users of simulation programs have one thing in common: their goal is to save cost through the virtual representation of reality, be it through component simulation to reduce cost-intensive testing on test rigs with equally cost-intensive prototypes, or through optimising the forming process on the computer, thus avoiding expensive machine hours and modifications on forging dies. The basic advantages, which in the end result in cost reduction and thus an improved competitive position for forged parts, are explained in more detail below. Shortening of process development The shortening of process development is achieved by using the computer rather than trial-and-error methods on expensive production machinery. In the ideal case no alterations or only marginal ones need to be made during the forging of samples. On the one hand this results in a time advantage as compared with conventional development processes and on the other hand it enables considerably more complex designs to be forged which only a few years ago were considered to be "unforgeable". Reduction of initial weight Especially in recent years the cost factor initial weight has gained in importance because of the dramatic increases in raw material prices. The introduction of weight-saving measures, whilst retaining the robustness and quality of the production process can hardly be imagined without simulation. Often there are other secondary effects, like the reduction of die stresses or die wear. Recognition of problem areas Through the introduction of powerful hardware and the parallel running of computer algorithms, the possibilities for the recognition of problem areas have been vastly increased. The high resolution of FEM-meshes (2 3 mm) possible today means that possible sources of failure can, with a high degree of probability, be found in advance. Nowadays this is not only the case for the formed parts themselves, but also for the dies, as in most systems this can be analysed with respect to loading and deformation. In any case, the recurring recognition of problem areas serves to train designers and process developers, which in turn leads to fewer sources of failure and thus also to a reduction in the number of virtual development iterations. Training of designers In textbooks one can often read, "forging is more an art than a science." This means that a process developer for forging and bulk metal forming processes can scarcely learn anything from textbooks, but can in fact only become a professional through his own development activity and the accompanying acquisition of experience. "Seeing through" the dies and the exact analysis of processes developed by oneself helps, along with the still indispensable learning-by-doing, to build up knowledge gained through experience more quickly than would be possible without simulation. Trials Competitive advantage can normally be defined by one's know-how lead compared to competitors. Alongside firms in the same industry, the main competition here can be companies using other technologies such as casting or sheet metal forming. In order to keep up in this competition, there are two basic methods available: Development of existing products with new methods and Development of new products with existing methods. Finally there remains the more unusual variant of combining these two ways. The costs involved in these important developments can be markedly reduced by the use of forming simulation and this reduces the threshold of inhibition connected with exploring new ways, which in turn serves to secure competitiveness. 9

11 Experience Reports Below, companies belonging to our user circle elaborate on, among other things, their experience with simulation in forging and bulk metal forming. CDP Bharat Forge GmbH As a developer and supplier of highly complex automotive components for all well-known OEMs, we at CDP Bharat Forge GmbH are focussed on utilising every modern aid available, especially in design and development, so as to be able to hold our own against steadily increasing competition. The present development chain for a forged wheel carrier is shown in Fig. 1. As opposed to the traditional method, the automobile manufacturer is involved at a very early stage. Development times are thus shortened and cost for additional expenditure reduced. Starting with the structural space allowed by the customer and with the specification, the most modern software is used in the simultaneous engineering development chain. An indispensable part of development today is the simulation of forming processes. System application Since the introduction of Forge3 in 1998, many functions have been added or have been further developed so that they are of economic interest. Among these features are, alongside the actual material flow simulation and stress analysis of the forging dies, such things as representation of grain-flow and of sheared surfaces, and the reverse simulation of so-called marking grids for the detection of surplus material and of faults (Fig. 2). For example, any area of the work-piece can be traced through the forming process. Equally it is possible to retrace any part of the finished product back to its original position in the initial stock (billet) used. Increasingly all processes which have an effect on quality or on properties, which take place either before or after the actual forming process, are being brought into the analysis. Fig. 3 shows examples of investigations into the formation of so-called edge-deformation formed during hot shearing of 140 mm square bars. A further integrated function is the stress analysis of dies. By using a simulation following on from the material flow, stress patterns (Fig. 4) and deformations can be analysed and the necessary countermeasures introduced. Coupled simulation (material flow and tool analysis in one simulation), which will provide considerably more accurate results than hitherto, is to be introduced shortly as a standard method. Experience with the system Because of the good results achieved, its computing speed, its range of functions and its good support, CDP Bharat Forge GmbH can report having good experience with Forge3. Even if, as before, wishes for improvements exist, its use now is justified by the considerable potential savings available. The enormous cost caused by weak points in the forging process justify the use of simulation. Besides the algorithms on which the system is based, the quality of the results presented in Table 2 are highly Bild 2: Rückverfolgung der Lage von Überschussmaterial (blau) im Grat zum Vorprodukt Fig. 2: Retracing position of excess material (blue) in flash through previous production stages Bild 3: Simulation des Kanteneinzugs an warmgescherten Abschnitten Fig. 3: Simulation of edge deformation of hot-sheared billets dependant on the user's qualifications and his physical understanding of the process. Distinct improvements in predicting of forging defects, fill-up of dies and forging force can sometimes be achieved using a new physicallybased friction model which calculates friction forces as a nodal variable, wheras this variable is computed locally and time dependent for each increment. The model has been implemented as a blackbox solution and requires no input Bild 4: Spannungs- und Deformationsanalyse eines Obergesenks (Schnitt) Fig. 4: Stress- and deformation-analysis of upper die-half (section) 11

13 Bild 5: Automatisch berechnete Reibung bei der Simulation eines Lkw-Achsschenkels (rot = hohe Reibung; blau = Minimalreibung) Fig. 5: Automatically computed friction values during simulation of truck steering knuckle (red = high friction; blue = minimal friction) from the user. An example can be seen in Fig. 5, which shows the progression of friction distribution during the forging of a truck steering knuckle. Furthermore, so-called project templates have been created for all standard products. These are centrally archived to achieve a further standardisation of pre-processing. The user must merely add the product geometry, press kinematics and material files. Alongside the optimisation directly in the process, in particular the learning or training effect must be mentioned. The designer today can check out his ideas directly in the simulation and has the possibility of looking inside the process. Since the introduction of a Windows cluster with nine nodes, larger projects with finer meshes have now become possible. With this system the probability of detection of possible faults during the very first calculation has increased markedly. Fig. 6 shows the mesh-partitioning of a reducerrolling model. In the past, even mesh-modules of 12 mm placed large demands on the system, whereas now 6 mm meshes can be computed within acceptable time spans. A comparison of the computing time needed for a passenger car carrier, using various hardware configurations, is shown in Table 3. The time required for pre- and post-processing depends very much on the application and on the aim of the simulation. The standard application takes between 2 and 16 hours for a complete project. Area of application Future expectations The development of software and hardware has made great strides in the last few years. Whereas a few years ago only FEM and computer experts were able to use the systems, nowadays suitably trained designers can run standard applications as part of their daily routine. A further improvement in economic aspects is however needed to reduce the time required for pre- and post-processing. Further optimisation is necessary to increase stability, to widen functionality and to improve the results for special areas of application. In particular Quality of the results / Remarks Forging fault 95 % Major faults are normally shown, minor faults only with a very fine (folds, cracks, etc.) mesh (mesh size less than or equal to size of fault!) Filling of the form 95 % Form-filling and flash formation very good, filling behaviour in more deeply die cavity only possible with special frictional models Grain flow and 95 % Very good shearing edges Forging force 90 % Relatively good (deviation of ± %) Die stresses 70 % Qualitative prediction very good, quantitative prediction not always conclusive (with special tool steel data and linked simulation also improved quantitative prediction) Wear 70 % Relatively good at present only qualitative prediction Table 2: Quality of calculation using Forge3 Application Computing time with Computing time with mesh size 8 10 mm mesh size 4 6 mm Cluster: Cluster: 1x3,5 2x2,2 9 x3,5 1x3,5 2x2,2 9 x3,5 GHz GHz GHz GHz GHz GHz Reducer-rolling 13 h 12 h 2,5 h 25 h 24 h 6h with 4 passes Upsetting 1 h 45 min 4 min 2 h 1,5 h 10 min Blocker/ 38 h 37 h 4h 67 h 64 h 8h Finisher Elastic die stress analysis (standard with only one processor) < 2 h Table 3: Average computing time for simulation of passenger car wheel carriers strength analysis and wear prediction for dies as well as a deeper penetration of the material physical aspects of grain-microstructure property relationships are to be seen as having priority. The possibility of calculating finer meshes using the cluster technique has increased the demand for powerful documentation and archiving technologies enormously. Data quantities of up to 10 Gbyte for one forming stage are no longer unusual and will in future become more common, taking into consideration that in the medium term coupled simulation as well as calculation of micro-structure evolution will be possible as a standard. 13

15 Hatebur Umformmaschinen AG Hatebur Umformmaschinen AG has been developing and marketing horizontal high-speed multistation presses for hot and cold forging for 75 years. The ability to develop, produce and, in the case of smaller machines, sometimes in-house trials. In this process, simulation software is used as a standard to establish the optimal layout of the forming sequence. In research and development, The results are always discussed and evaluated with practical experts as a team. Normally the following points are investigated: Material flow, achievable geometry, estimation of forming loads and resulting stresses (Fig. 8). More rarely: Tool loading, stress conditions in critical tools (Fig. 9), Grain flow, tracing of particular points on the formed part, Checking for laps and cracks (Fig. 10). Experience with the system The experience gained has been for the great part highly positive. It has been shown that it is not the actual use of the system which is exacting, but rather the interpretation of the results. In order to improve the certainty and accuracy of the results, real-life results are compared as often as possible with the simulation. This is of great importance, especially with regard to the forming load, as the two can diverge considerably (e. g. when the filling degree of the part differs just slightly between theory and practice). Particular uncertainties and inaccuracies can occur in the calculation of processes which Bild 9: Werkzeugbelastungen in einem Armierungsverbund bei geschmiedeten Fließpressteilen (links), Zone mit starkem Verschleiß (Mitte), Verschleiß am realen Werkzeug (rechts) Fig. 9: Die stress in armouring kit for forged extrusions (left), high wear zone (centre), wear of actual die (right) also do in-house testing of forming processes for our own machines is an important reason for our being able to act in the market as a partner. As well as experience, the use of modern tools is an important element in the development of multistation forming processes. For this reason, Forge2 and Forge3 have been in use since 1999 for process development as well as for judging the evaluation of the feasibility of parts in advance of machine projects. System application At Hatebur the simulation software is used for the calculation of cold and hot forming processes. This happens mainly in the following cases: At the start of a machine project, the use of simulation software can be justified when a process involving new types of formed parts or using materials at unusual temperatures is requested, as well as for the documentation and verification of complex production stages. Tool development for Hatebur multistation presses includes the development, manufacture and simulation software not only allows the analysis and optimisation of new types of process or process steps, but it can also be made easily understandable for outside persons (Fig. 7). are not done daily. Here the time or the experience to acquire the necessary parameter (correct input parameters such as heat transfer rates, friction coefficients, etc.) is often lacking. Future expectations The handling of the software must allow more efficient utilisation in daily use together with greater reliability of the results. Data input and the data basis (flow graphs, friction coefficients, material data. etc.) must be so organised that calculations can be performed simply over the whole temperature range and without further work on fundamentals. Bild 10: Kontrolle auf Falten anhand des Faserverlaufs Fig. 10: Checking for laps using fibre orientation 15

17 Hirschvogel Automotive Group Bild 12: Faserverlauf (links stehende, rechts liegende Schmiedung) Fig. 12: Fibre orientation (left vertical, right horizontal forging) can be adequately represented by FEM. This goes so deep into the layout process for components that with the aid of simulation an optimal fibre orientation can be found. As is well known, components whose fibre orientation runs in the direction of highest loading demonstrate the highest resistance to failure. Here the possibility occurs of already deciding at the design stage on the forging process to be used after having carried out a material flow simulation including a fibre orientation investigation (Fig. 12). Bild 13: Performance Windows-Cluster Fig. 13: Performance of Windows cluster The intelligent development of a process chain which is optimal from both the production and the economic viewpoint has nowadays become more important than ever. The classic forming of simple raw parts to drawing is losing more and more in significance. Instead, demanding, machined components ready for assembly with assured properties are required ever more often. This requires both the use of virtual tools during the process layout and the combination of several different processes (e. g. warm and cold forging) to do justice to the increased requirements. Furthermore, it can be seen that component development is being moved ever more in the direction of the supplier. The Hirschvogel Automotive Group has accepted this challenge and is already delivering ready-to-assemble components, including the whole development process and can, if required, offer static and dynamic component testing (Fig. 11). System application In the meantime, simulation of material flow has found a place in the normal course of creating process design. This, by the way, is no longer the case just for 2D simulations of axisymmetrical components, but equally unrestrictedly for complex 3D forming processes. Finding the ideal preform is nowadays unproblematic, and even more demanding processes Experience with the system In spite of continuous improvement in performance of the hardware available, a massive expansion of computing capacity in the Hirschvogel Automotive Group was required. As an additional task, the consideration of the multi-user environment had to be solved, i. e. to give every user equal access, despite spatial separation, a batch management system had to be implemented. The solution takes the form of a Windows cluster, which offers greatly increased computational power as compared with standard hardware. Nevertheless, the efficient use of this enormous computational power has large consequences for the organisation. The engineer profits from the markedly reduced computing times as the computational task can be carried out by several PCs simultaneously. When the calculation of a large model, such as often found with chassis parts, is started the evening before, the results are available the next morning. In comparison, calculation times of up to a week were quite usual for similar components in the past (Fig. 13). The application of FEM is nowadays no longer exclusively limited to the conventional forging process, but more and more the periphery is moving into the foreground. In this way also the stresses and the associated deformations in the die are increasing in importance (Fig. 14). 17

19 In the meantime, even the distortion of the forms in the dies can be considered, so as to finish up with dimensionally correct parts (Fig. 15). Future expectations To increase their power of innovation, the Hirschvogel Automotive Group has always taken part in a series of research projects as an industrial partner. For example, in the context of a research project the attempt is being made to take the behaviour of the forging machine into account in the simulation. The aim here is to determine the slant of the ram by linking forming and machine simulations, taking into account guide clearance and machine stiffness (Fig. 16). Elastic component simulations are already routine at Hirschvogel. In future the customer can also be supported by the calculation of so-called "failure cases", where plastic deformation plays a major role (Fig. 17). Ever more software tools in the business of bulk metal forming are indeed coming into use, but neither developer nor user can be completely satisfied with what has been so far achieved. Further projects combining research institutions and industry will be necessary to eradicate the remaining deficits in creating virtual process layouts. For the description of friction in the workpiece-die-interface, for example, there is still no solution in sight. In just the same way, the prediction of material failure in coldforming provides results which are too inaccurate to quantify the point where the formability of the material will be exhausted. One further potential topic for simulation is the computation of yield-stresses based on micro-structure simulation. Summing up, the ultimate common goal of the German forging Bild 16: Gekoppelte Simulation Umformprozess und -maschine Fig. 16: Linked simulation of forming process and forming machine Bild 17: Elastisch-plastische Bauteilsimulation Fig. 17: Elasto-plastic structural analysis industry can be expressed as follows: we should be able to represent on the computer the entire process chain, from raw material to ready-to-assemble component, including all relevant properties and characteristics, so as to ensure that our competitiveness can be maintained in the future through our technological lead. 19

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